A typical example of the alunite type compound is alunite. Natural alunite is existent as alumstone in thermal deposits and acid corroded districts formed by active volcanoes and hot springs. Synthetic alunite is industrially used as an adsorbent, additive for resins, filler or carrier. The following synthesizing methods are known.
In the specification of the present invention, documents 1 to 6 mean the following.    document 1: Kohno et al., “Magazine of Mineralogy”, vol. 20, Nos. 1 and 2, pp. 13-23, January and April, 1991    (2) document 2: Inoue et al., “Bulletin of the Japan Society of Chemistry”, No. 2, pp. 156-162, 1985    (3) document 3: JP-A 64-11637    (4) document 4: JP-A 64-11638    (5) document 5: JP-A 2000-7326    (6) document 6: JP-A 6-122519
Document 1 discloses a method of synthesizing alumstone by mixing aluminum sulfate, potassium sulfate and sodium sulfate in a fixed ratio and stirring them at 100° C. under atmospheric pressure for 48 hours.
Document 2 discloses a method of producing alunite having a specific surface area of 200 to 240 m2/g by adding potassium sulfate and potassium hydroxide to an aqueous solution of aluminum sulfate so as to adjust the K/Al ratio to 5 and pH to 3.7 and boiling and refluxing the resulting mixture for 3 hours. It is reported that the alunite produced by this method contains slit-like pores with a diameter of 14 Å and a width of 30 Å, has water absorptivity equivalent to that of silica gel and high SO2 and NO absorptivity, and adsorbs an acid dye well.
As means of manufacturing alunite compounds at a low cost and high yield so as to use them as adsorbents in industrial fields, manufacturing processes disclosed by documents 3, 4 and 5 are known.
Document 3 discloses an alunite type adsorbent which has a BET specific surface area of 280 m2/g or more and a total volume of pores having a diameter of 10 to 300 Å of 0.05 ml/g or more and is represented by the following formula:MM′3(SO4)2(OH)6 wherein M is a monovalent cation, and M′ is Al or a combination of Al and Fe(III).
For the synthesis of the adsorbent, document 3 teaches a method for crystallizing an alunite type lamellar compound having an increased specific surface area by maintaining pH of a reaction solution at 4.0 to 4.4 from the beginning of a reaction and at not less than 3.8 during a reaction when aluminum sulfate or a combination of aluminum sulfate and ferric sulfate is thermally reacted with an excessive amount of an alkali sulfate in an aqueous solvent containing an alkali hydroxide.
Document 4 discloses an adsorbent composition which has a BET specific surface area of 300 m2/g or more and a pore volume of 0.1 ml/g or more and is an homogeneous composition comprising a lamellar compound having a chemical structure represented by MM′3(SO4)2(OH)6 (M is a monovalent cation, and M′ is Al or Fe (III)) and an alunite or jarosite type crystal structure and 5 to 80 wt % based on the lamellar compound of amorphous silica or amorphous silica alumina. As for the manufacturing process of the adsorbent composition, document 4 teaches that alunite and jarosite type lamellar compounds can be crystallized according to starting materials and the value of pH during a reaction.
Document 5 discloses an alkali aluminum sulfate hydroxide whose individual particles are independently spindle-like or spherical and which is represented by MAl3(SO4)2(OH)6 (M is a monovalent alkali metal or ammonium group) and specified by its chemical composition that satisfies 1.2≦D25/D75≦2.0 (D25 is the particle diameter of particles which account for 25% of the total and D75 is the particle diameter of particles which account for 75′ of the total in a volume-based cumulative particle size distribution curve measured by a Coulter method), X-ray diffraction image different from that of alunite, pH of a 5% aqueous suspension, BET specific surface area and moisture absorptivity. Further, document 5 proposes an alkali aluminum sulfate hydroxide which has ideal parameters such as particle bulk density, volume-based median diameter, sharpness of particle size distribution, aspect ratio, refractive index and abrasion when it is added to a resin. As for its manufacturing process, document 5 teaches that that aluminum sulfate, alkali sulfate or ammonium sulfate and aluminum hydroxide are hydrothermally treated. Document 5 also suggests a method of controlling the shape of each particle to a spherical or spindle-like shape by the content of alumina in a reaction system.
The D25/D75 ratio of the particles actually obtained in document 5 is in the range of 1.45 to 1.61.
Meanwhile, document 6 discloses “jarosite particles (amorphous hydrous ferric oxide particulate powders)” which are spherical, have an average particle diameter of 3 to 30 μm, a BET specific surface area of 150 to 300 m2/g and a bulk density of 0.7 to 1.1 g/ml and are represented by RFe3(SO4)2(OH)6 (R is K+, Na+, NH4+, etc.) as well as a synthesizing method thereof. Document 4 proposes a method of synthesizing jarosite particles by letting an oxygen-containing gas pass through a mixture of an aqueous solution of ferrous sulfate and an aqueous solution of a sulfate of an alkali metal or ammonium ion to carry out an oxidation reaction at a temperature higher than 45° C. and lower than the boiling point.